1. Field of the Invention
The present invention relates to a system for generating a video display and in particular to a system for gracefully aging unused areas of a video display.
2. Description of Related Art
As illustrated in FIG. 1, the HDTV (high definition television) television monitor displays a video image with a 16:9 aspect ratio (width to height) while older television monitors display video images with a 4:3 aspect ratio as illustrated in FIG. 2. Since most video sources continue to use the traditional 4:3 aspect ratio, it is necessary to make a compromise when processing a video signal conveying 4:3 aspect ratio video content to produce a display on a 16:9 aspect ratio HDTV video monitor.
As shown in FIG. 3, one approach is to stretch the 4:3 aspect ratio image horizontally to fill the 16:9 aspect ratio display area, but this distorts the image. Another approach is to clip the top and/or bottom of the 4:3 aspect ratio image to fit in the 16:9 aspect ratio display monitor as illustrated in FIG. 4, but this can cause an objectionable loss of part of the image.
One common approach, as shown in FIG. 5 is to display the image with the correct 4:3 aspect ratio in an active region 12 of 16:9 ratio display monitor, leaving areas 10 and 11 to the left and right of the active display area 12 unused. The unused screen areas 10 and 11 are usually made white as illustrated in FIG. 5, black as illustrated in FIG. 6, or some constant shade of gray. When the HDTV monitor is based on CRT (cathode ray tube) technology, such long term display of an unchanging image in the unused areas 10 and 11 of the display causes electron beams to continuously strike the same phosphor pixels on the screen, and they can eventually become noticeably less bright than phosphor pixels in the active area of the display. This phosphor burn-in produces a permanent—ghosting—in the unused areas 10 and 11 of the video display that will be visible when viewing 16:9 aspect ratio video content.
A similar problem arises in the context of a CRT personal computer monitor, which can suffer phosphor burn-in when left for long periods displaying an unchanging image. A screen saver resolves this problem by displaying a moving pattern on the monitor upon detecting that the monitor has displayed an unchanging image for some minimum period. When a monitor displays a moving pattern, electron beams will strike each pixel on the screen only sporadically and somewhat randomly so that all phosphor pixels on the screen tend to age more evenly and gracefully, thereby preventing screen ghosting.
Various approaches have been used for solving the uneven display aging problem in televisions and other video applications in which only a part of the display area of a monitor is used to display moving video images, as shown, for example in FIGS. 5 and 6. U.S. Pat. No. 6,359,398 issued Mar. 12, 2002 to Nakajima et al teaches to prevent uneven phosphor aging by duplicating within the inactive areas of a display, a portion of the video source shown in the active area of the display. This does prevent uneven phosphor aging, but it produces an unpleasant double-vision effect in the display. U.S. Pat. No. 5,291,295, issued Mar. 1, 1994 to Srivastava, teaches a method for equalizing the phosphor aging wherein unused areas to the right and left of an active display area are each divided into three sections. A video processing system then adjusts the illumination level of each section as a function of the illumination levels of adjacent areas of the active portion of the screen. U.S. Pat. No. 6,429,894 issued Aug. 6, 2002 to Hicks teaches to prevent uneven phosphor aging by using a sensor to monitor the illumination levels and/or other aging indicators in the active area of a display while a monitor is in use, calculating an amount and time to illuminate inactive areas of the display needed to compensate for the aging of the active area, and then illuminating the inactive areas while the monitor is not in use. This method allows the inactive areas of the display to remain dark while the monitor is being used, but it requires a sensor external to the monitor and requires the monitor to be left on when not in use so that the aging system can age the inactive areas of the display, thereby increasing energy consumption. U.S. Pat. No. 5,193,006, issued Mar. 9, 1993 to Yamazaki teaches to reduce ghosting by monitoring the brightness level of the active portions of a CRT display using an automatic brightness limiter driven by a fly back transformer and adjusting the brightness level of the inactive areas of the display to be approximately equal to the average brightness in the active area of the display. Alternatively, Yamazaki teaches to continuously move the active area in the display so that no area of the display remains continuously inactive. This helps to smoothes phosphor aging over the entire display but areas of the display near its edges still age differently from areas of the display near its center, since the center areas of the display are always active. U.S. Pat. No. 6,262,772 issued Jul. 17, 2001 to Shen et al teach to eliminate ghosting by stretching the active area to fill the entire display as illustrated, for example in FIG. 3. However, this method distorts the displayed image.
What is needed is a system for gracefully aging phosphors in inactive areas of a video display in a way that does not distort displayed images with producing distracting or unaesthetic images in the inactive areas, and without requiring the use of sensors external to the monitor.